I simply wasn't doing it in the right place (trying to use it in the $template array in functions.php).
I added:
'S_IN_PORTAL' => true,
to the $template array inside portal.php, and now it works just fine.

***/EDIT***

OK.
I'm really sorry that I'm a nub poster, and for most of you my question is stupid-simple, but I've been trying to get this to work for ~14 hours now.
The worst part is, all I want to do is get a stupid true/false response to an argument defined in a page.
Everywhere I look, though, all the examples and explanations go into 3-12 layers of abstracts, when I just want to get a simple true/false response.
So, yeah, eh? I'm frustrated.

Anyway.
On to the question.

I installed portal.php on my phpBB3 install (not quite in the default locations), and in order to get my breadcrumbs showing how I wanted (showing forum root) I simply inserted a {U_INDEX}->{L_INDEX} link before the <!-- BEGIN navlinks -->.

Now, however, I want my breadcrumbs to display like so:
Home -> Forums -> Forum_1 -> SubForum_2 (or if you prefer)
{L_PORTAL} -> {L_FORUM} <navlinks>
***Except***
I do not want {L_FORUM} to show if I'm on the portal.php (home) page.

So, portal.php has the define:

define('IN_PORTAL', true);

So I just thought I would use that as a definer and pass if NOT in portal -> show this.

Which means I need to define "S_IN_PORTAL" in either portal.php or functions.php, correct?

Except for the life of me I can't figure it out.
And, as I wrote at the start, trying to find a simple boolean pass is like asking for "just black coffee" at starbucks.